Distinct 29Si MAS NMR peaks from Si-Al permutation on neighboring T sites of unequal Si-O-T angles: Direct evidence from J-resolved experiment on K-cymrite (KAlSi 3O 8·H 2O)

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    Abstract

    29Si MAS NMR is a well-established tool for quantitative characterization of Si-Al distribution in aluminosilicates. In framework aluminosilicates, all tetrahedrally coordinated Si/Al (T) are linked via bridging oxygens to four other Si/Al (Q 4). Each crystallographically unique T site is commonly assumed to give at most five 29Si NMR peaks with chemical shift spacing around 5 ppm, corresponding to five Si units each linked to nSi and (4 - n)Al (n = 0-4), abbreviated as Si(nSi) hereafter. Here we report a detailed one- and two-dimensional (2D) 29Si NMR study on K-cymrite (KAlSi 3O 8·H 2O), which possesses a double-layered structure with all Si/Al distributed in one crystallographically unique T site (Q 4). Contrary to general belief, more than five 29Si MAS NMR peaks were resolved. These peaks are attributable, via 2D J-coupling mediated experiments, to Si(nSi)(n = 0-4), with two peaks each (∼2 ppm spacing) for n = 1-3. The latter corresponds to different permutations of the nSi(4 - n)Al over neighboring T sites of two distinct Si-O-T angles (139 and 180°). This suggests for the first time that the 29Si chemical shift varies with Si-O-Si and Si-O-Al angles differently. Si-Al exchange over neighboring T sites of unequal Si-O-T angles is another possible cause for multiple 29Si NMR peaks.

    Original languageEnglish
    Pages (from-to)10714-10722
    Number of pages9
    JournalJournal of Physical Chemistry C
    Volume116
    Issue number19
    DOIs
    Publication statusPublished - May 17 2012

    ASJC Scopus subject areas

    • Electronic, Optical and Magnetic Materials
    • Energy(all)
    • Physical and Theoretical Chemistry
    • Surfaces, Coatings and Films

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